1. Field of the Invention
The present invention is generally related to a method of etching or cleaning off native or other silicon dioxide (SiO.sub.2) from the surface of a silicon (Si) substrate and, more particularly, to a safe method of etching or cleaning off SiO.sub.2 which employs anhydrous ammonium fluoride (NH.sub.4 F) as a source of hydrofluoric acid (HF) which is used for etching or cleaning off the SiO.sub.2.
2. Description of the Prior Art
If the surface of a silicon substrate is exposed to ambient air, a native SiO.sub.2 layer forms on its surface. The native SiO.sub.2 layer presents certain manufacturing difficulties since many semiconductor processes, such as low-temperature epitaxy, polysilicon deposition, and silicidation, require the silicon wafer surface to be free of all native SiO.sub.2. Over the years, many different processes for etching or cleaning off the native SiO.sub.2 from Si surfaces have been developed. One of the most commonly used techniques for etching SiO.sub.2 is to expose the substrate to HF (wet or dry). However, the use of liquid or vapor HF has major drawbacks for the semiconductor manufacturer because it is both highly toxic and highly corrosive. The high toxicity of HF requires the semiconductor manufacturer to implement safety procedures and devices which assure that personnel and the environment are not injured by the HF gas. The high corrosiveness of HF requires the semiconductor manufacturer to utilize non-corrosive storage containers and non-corrosive tubing for delivering the HF gas to a cleaning or etch chamber, and this type of equipment is generally more expensive than containers and tubing that do not have non-corrosive properties.
In U.S. Pat. No. 5,030,319 to Nishino et al. and in the article to Nishino et al., 1989 Dry Process Symposium IV-2, 90-92 (1989), a method of selectively etching native SiO.sub.2 from a silicon substrate is disclosed which employs the fluorine atoms and nitrogen hydrides produced by an NH.sub.3 and NF.sub.3 microwave discharge. In the Nishino et al. etch process, a film of (NH.sub.4).sub.2 SiF.sub.6 is formed by condensation on the surface of the substrate by the reaction of NH.sub.4.sup.+ and F.sup.- with the polarized Si atom of the ionic SiO.sub.2 species. For film formation to occur, the substrate temperature must be maintained below 100.degree. C., and is preferably performed at room temperature. After film formation, the substrate is heated above 100.degree. C. and the (NH.sub.4).sub.2 SiF.sub.6 film vaporizes to leave an oxygen free silicon surface.
The Nishino et al. etch process is not ideal because it requires the transport of corrosive and toxic gases into a reaction chamber; therefore, the cost of non-corrosive tubing and storage chambers as well as the requirement of increased safety procedures are not avoided by the Nishino et al. etch process. Moreover, the Nishino et al. process imposes timing and temperature control parameters on a semiconductor manufacturer since the wafer temperature must first be maintained below 100.degree. C. to allow an etching film to form on the substrate surface, and subsequently, the substrate must be heated above 100.degree. C. to remove the etch products. Nishino et al. acknowledge that if the temperature is kept above 100.degree. C. initially, film deposition does not take place and no SiO.sub.2 etching occurs.